In preliminary work the PI and his undergraduate coworkers have discovered a small molecule that binds to CD26 at a site separate from the active site; binding of the small molecule alters the protease activity. This small molecule is a constrained dipeptide containing proline where the proline amide is constrained to the cis conformation. It is suggested that a cis-proline is a recognition element for a ligand binding site on CD26. Structural relatives of the constrained cis-proline dipeptide will be synthesized; then their interaction with CD26 will be examined using an enzyme assay.